1. Field of the Invention
The present invention relates to a torque transmission device of a four-wheel drive vehicle for transmitting torque, as interposed between a pair of drive shafts of the four-wheel drive vehicle.
2. Description of the Prior Art
A four-wheel drive vehicle is not only excellent in running properties on rough roads, but also excellent in accelerating properties and running stability on general roads. Accordingly, such a four-wheel drive vehicle is now rapidly widespread among the general public.
There is known a four-wheel drive vehicle in which the front-wheel drive shaft is rigidly connected to the rear-wheel drive shaft. In this arrangement, however, the propeller shafts are twisted if there is a difference in the number of rotations between the front wheels and the rear wheels due to the difference in turning radius between the front wheels and the rear wheels at the time when the vehicle travels at a corner. Further, the arrangement above-mentioned presents the problem of a so-called tight corner braking phenomenon that the rear wheels presenting a smaller turning radius are dragged as slipping, causing the vehicle to become shaky. To prevent the problems above-mentioned such as the tight corner braking phenomenon and the like, a conventional four-wheel drive vehicle is provided between the drive shafts with a torque transmission device which can transmit torque to both drive shafts while allowing the difference in rotational speed between both drive shafts.
As the torque transmission device above-mentioned, there are proposed a so-called viscous coupling, a device of the hydraulic pump type using a vane pump, and the like.
Disposed in the viscous coupling are a plurality of first clutch discs rotatable together with one of the drive shafts and a plurality of second clutch discs rotatable together with the other drive shaft, first and second clutch discs being alternately disposed as coming in close contact with each other. Both first and second clutch discs are hermetically sealed with highly viscous oil interposed therebetween. In a normal state, both first and second discs are coupled to each other by a fluid frictional power of the discs with the oil and a shear force inside of the oil, so that torque is transmitted. When the difference in rotational speed between both first and second discs becomes great, the oil is stirred and thermally expanded. This causes both first and second discs to be pressingly contacted with each other, so that both drive shafts are securely coupled to each other to suddenly increase the torque (a so-called hump phenomenon occurs).
As the hydraulic pump device above-mentioned, there is known a device using a vane pump formed such that a rotor having vanes rotatable together with one of the drive shafts is coaxially disposed in a cam ring of a casing rotatable together with the other drive shaft. In the vane pump, the rotor and the cam ring are coupled to each other through pressurized oil interposed therebetween, so that torque is transmitted to both drive shafts. Theoretically, the torque thus transmitted is greater as the pressure generated by the vane pump is higher; that is, the difference in rotational speed between the both drive shafts is greater.
In a viscous coupling, the hump phenomenon above-mentioned enables both drive shafts to be securely coupled to each other. Accordingly, the viscous coupling is advantageous in that it can be incorporated in a differential gear for example and used as a limited slipped differential gear for directly coupling the right- and left-wheels to each other as necessary. However, such a viscous coupling presents the problem of durability that, by an explosive increase in inner pressure at the time when the hump phenomenon takes place, the seals are broken and the oil among the plates is deteriorated by shear.
In a device of the hydraulic pump type, the torque is transmitted by an inner pressure force of the oil, and not by its shear force. Accordingly, such a device is advantageous in view of durability because the oil is hardly deteriorated. However, when the pressure generated becomes high, the side plates which are disposed at both end surfaces of the cam ring and which form a pressure chamber of the vane pump, are deformed to cause the gap between the rotor and the side plates to become great. This restrains the generated pressure from being increased, so that the transmitted torque cannot be sufficiently increased for the increase in difference in rotational speed between both drive shafts. To overcome the problem above-mentioned, it may be proposed to thicken the side plates to increase the rigidity thereof. However, an available space is often limited to make it difficult to make such provision.
To overcome all the problems of the both systems above-mentioned, there is proposed a torque transmission device in which a multiple disc clutch is axially incorporated in the device of the hydraulic pump type above-mentioned with a plurality of plates of the clutch being adjacent to one another, and in which the multiple disc clutch is adapted to be operated through a clutch operating member by a cam mechanism so that the both drive shafts are securely coupled to each other, when the difference in rotational speed between both drive shafts is increased (that is, when the torque transmitted by a hydraulic pump is increased) (Publication for Japanese Patent Unexamined Application 262730/1991).
In this torque transmission device, the cam mechanism is disposed between the housing of the torque transmission device and the casing of the hydraulic pump. The entire hydraulic pump is axially moved by the cam mechanism, and the multiple disc clutch operating member is pushed through the hydraulic pump. Accordingly, a great load is applied to the casing of the hydraulic pump. It is therefore necessary to provide the casing with sufficient strength. This disadvantageously increases the production cost and the sizes of the hydraulic pump. This subsequently causes the torque transmission device to become large-sized.